The present invention relates to a hydraulic servo brake system with slip control for automotive vehicles, with at least one brake circuit connected to the brake pressure chamber of a master cylinder and/or hydraulic power booster, with a slip monitoring and control device, with supply and discharge valves within the brake circuit for the control of the wheel cylinder pressures during slip control and with an auxiliary valve with switches over at the beginning of slip control. The auxiliary valve activates a throttle point within the brake circuit on the supply side and is switchable, in particular, in opposition to the force of a return spring by means of a control pressure occurring during slip control.
In a known servo brake system of this type (German printed and published patent application No. 32 40 404), the auxiliary valve is a slide valve which is common to all brake circuits and which has a plurality of change-over portions which are, in each case, arranged upstream of a supply valve. When the control pressure occurs, the slide is being displaced in opposition to the force of a return spring, that is, from a first position in which the passage is effected by way of a larger-cross section annular groove each, into a second position, in which the passage is effected by way of a smaller-cross section annular groove, that is a throttle point. The electromagnetically actuated supply and discharge valves are, in each brake circuit, united to form a slide valve.
Since brake pressure fluid is transmitted to the wheel cylinder by way of throttle point during the control action, the wheel cylinder pressure increases only gradually when the brakes are supplied, just as it is desired for a satisfactory slip control. However, up to the moment at which the slip monitoring and control device responds, the supply of brake fluid to the wheel brake is throttled considerably less, thus enabling a quicker pressure increase in the wheel cylinder.
The present invention has as an object to create a hydraulic servo brake system of the type initially referred to which it is possible to achieve a greater change of the gradient of the wheel cylinder pressure when the auxiliary valve switches over.